1. Field of the Invention
The present invention relates to a small-sized, high power amplifier, and more particularly to an amplifier applied to a wireless apparatus, such as a portable terminal, which is operated by a battery.
2. Description of the Related Art
Recently, in a conventional high power amplifier used in the final output stage of a wireless transmitter, characteristics of a high efficiency, a compact size and a low price have become necessary for promoting relatively lower power consumption from the viewpoint of an entire wireless apparatus. Generally, in the conventional amplifier, efficiency decreases at lower output power. In order to constantly maintain high efficiency over a wide range of output powers, it is essentially required to improve efficiency in the lower output power range.
As solutions to this problem, conventional methods include a method for controlling a power source voltage in a high power amplifier and a “bypass method” for switching and bypassing a final stage amplifier which consumes the most electrical power among all the amplifiers.
In order to increase efficiency of the power output, the first conventional method uses a DC-DC converter to control the power source voltage based on an output power.
FIG. 1 illustrates a circuit structure for performing the bypass scheme that is the second conventional method. Referring to FIG. 1, two amplifiers and two switches are connected with each other in a tandem connection. In case of a high power output, all of the two amplifiers 1 and 2 in the first and the second stages operate, whereas in case of a low power output, the amplifier 2 of the second stage is bypassed and turned off to improve the efficiency of power.
The first conventional method is disclosed in Japanese Patent Publication No. 2000-513544 (the primary reference). The primary reference describes that the DC-DC converter is used for controlling the power source in order to realize the first conventional method of achieving efficiency. The DC-DC converter, however, with the peripheral circuits thereof causes the entire apparatus to become bigger in volume as well as to be expensive. Furthermore, the first conventional method has a problem in that the conversion loss of the DC-DC converter tends to increase (i.e., efficiency is lowered) when the output power is lowered (i.e., the power source voltage is lowered). As a result, the structure of combining the amplifier and the DC-DC converter has a problem in the deterioration of the efficiency.
In the second conventional method using the bypass scheme, when the lower power is output, the efficiency is deteriorated to the extent of the power losses of the two switches, as shown in FIG. 1. Each of the two switches, connected to an output of the amplifier 1, generates a certain amount of loss, respectively. Also, the structure for the second method includes the switches that essentially cause the entire structure to increase in both size and cost.